1. Field of the Invention
This invention relates to a control system for a static var compensator adapted for performing phase control of a current of a reactor for each phase by a thyristor and thus controlling reactive power.
2. Description of the Related Art
It has been known that, in a static var compensator (hereinafter referred to simply as “SVC”) that performs phase control of a reactor current for each phase by a thyristor and thus controls reactive power, a harmonic current is generated when the magnitude of a reactor current is phase-controlled by a thyristor converter. For example, R. Mohan Mathur and Rajiv K. Varma, “Thyristor-based FACTS controller for electrical transmission systems”, published by Wiley-Interscience, 2002, p. 47-55, FIG. 3.9 shows the generation of a harmonic current. Since a tertiary harmonic current generated from TCR (thyristor controlled reactor) is large, outflow of the tertiary harmonic current toward the system is restrained by delta-connecting the TCR, as shown in FIG. 3.11 of R. Mohan Mathur and Rajiv K. Varma.
When the power system voltage is in a three-phase balanced state, the tertiary harmonic current is canceled by the delta connection and hardly flows out toward the system. However, when the system voltage includes a three-phase unbalanced component, the tertiary harmonic current is not canceled and flows out toward the system. When the power system has a resonance characteristic near the tertiary level, the tertiary harmonic current flowing out from the TCR expands the tertiary harmonic voltage distortion of the system. This causes problems of obstruction to sound operations of the SVC and adverse effects on the other devices connected to the system.
To deal with such cases, traditionally, in the example disclosed in R. Mohan Mathur and Rajiv K. Varma, a tertiary harmonic filter is connected parallel to the TCR to absorb the outflowing tertiary harmonic current.
R. Mohan Mathur and Rajiv K. Varma, and JP-A-61-221533 (FIG. 1 and its description) also disclose techniques for controlling the current of each phase of the TCR in order to equalize the magnitude of fundamental wave currents of the respective phases of the TCR (in JP-A-61-221533, it is described as SVC instead of TCR). However, JP-A-61-221533, and R. Mohan Mathur and Rajiv K. Varma disclose that simply equalizing the magnitude of the fundamental wave currents is not effective and may conversely expand the tertiary harmonic current. This is because the quantity of the fundamental wave current and generated tertiary harmonic current of the SVC has a nonlinear characteristic as shown in FIG. 4 and the polarity of a tertiary harmonic generation sensitivity coefficient K3 with respect to changes in α is reversed at a certain phase angle α0, thereby destabilizing the control.
In the traditional SVC, since the tertiary harmonic current flows out toward the system when the system voltage is unbalanced, there is a problem that the tertiary harmonic voltages is distorted and adversely affects the other devices connected to the system. Also, since the tertiary harmonic filter must be provided, there is a problem of increase in cost.
There is still another problem that if the currents of the three phases are controlled to be equal in order to restrain the tertiary harmonic current, the control becomes unstable and conversely expands the tertiary harmonic.